The manufacture of particulate fertilizers is generally carried out by agglomeration or prilling processes whereby particulates such as prills or granules of the fertilizer are formed. There are numerous agglomeration and prilling processes known for the manufacture of fertilizers such as ammonium nitrate, urea, potash, phosphates and mixtures thereof. Agglomeration processes are generally carried out in paddle mixers, pug mills, rotary drums, pans or fluidized beds. Prilling processes are usually carried out in prilling vessels such as prilling towers.
In many cases it is necessary to cool the particulates so formed to a reasonably low temperature to avoid undesirable phenomena such as caking. The cooling of particulates has been conventionally carried out with cooling air in a cooling vessel such as a rotating drum or a fluidized bed. Because of the intense motions and turbulence of the particulates in such vessels, the particulates are subject to attrition and abrasion that result in a breakdown of the particulates with the formation of fines and dust. This breakdown is highly undesirable. Not only does the formation of fines and dust reduce the efficiency of the above-mentioned processes, but the additional treatments that are necessary for the removal of fines and dust from the product add considerably to the costs. The fines and dust must be screened from the product, and the cooling air, which contains a portion of the dust, must be treated for the substantial removal of the dust before the air can be returned to the atmosphere.
In order to reduce attrition and abrasion it would be necessary to avoid the intense motion and turbulence during cooling. Such intense motion appears to have been somewhat reduced in the cooler for hot lime disclosed in U.S. Pat. No. 3 397 460. The cooler includes an indirect heat exchanger comprising a plurality of horizontal air ducts mounted in tiers to form vertical passages for the lime, the tiers being connected with transfer boxes, and a direct heat exchanger for cooling lime received from the passages with air. This cooler and its tiers of interconnected air ducts is of a rather complex construction and the free falling material and the direct heat exchanger still would cause considerable motion with resultant attrition.
In heat exchange between two liquids, or liquids and gases, use is often made of one or more spaced heat exchanger plates over which one fluid flows and through which the cooling fluid passes in indirect heat exchange. Such plates are used in vaporizers, dryers, heaters, condensers and coolers, and are based on disclosures such as made in U.S. Pat. No. 3 458 917. A heat exchanger plate according to this patent consists of two flat plates that are circumferentially seam-welded together, are provided with a pattern of spot or seam welds or both and are expanded between welds by introducing high-pressure fluid into the spaces between welds. The expanded plate has a smooth outside surface with numerous small pillow-like embossments between spot welds. The pillows are hollow and interconnected to provide flow channels for a cooling fluid. The pattern of seam welds may provide a serpentine passage for the cooling fluid through the plate. An inlet and an outlet for the fluid are attached to the plate. According to a catalogue of the Paul Mueller Company, the plates are used in various shapes and configurations, one of those being in packs of spaced plates with common inlet and outlet headers. No mention is made of cooling solids or particulate materials.